The regulations on particulate matters (PM), nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbon (HC), etc. emitted from diesel engines have been strengthened year after year. With the strengthening of the regulations, improving engines is no longer enough to address the regulations. Accordingly, techniques have been developed and employed for reducing these materials emitted from an engine. In the techniques, an exhaust gas passage is provided with an exhaust gas post-treatment device using catalysts.
As shown in FIG. 4, a conventional NOx purification system 1X including a selective reduction NOx catalyst (SCR catalyst) 7 for reducing NOx is provided with the selective reduction NOx catalyst 7 and an ammonia solution supply unit 6 upstream thereof. The ammonia solution supply unit 6 is a device for feeding ammonia (NH3) to the selective reduction NOx catalyst 7 and supplies an ammonia solution, such as aqueous urea, that becomes an ammonia source, into the exhaust gas G in an exhaust gas passage 3. The urea fed into the exhaust gas passage 3 produces ammonia by being hydrolyzed by heat of the exhaust gas G, or hydrolyzed through the hydrolyzing function, given to the ammonia selective reduction NOx catalyst 7, using heat and steam in the exhaust gas G. In some cases, alternatively, urea is converted into ammonia by a hydrolysis catalyst provided between the ammonia solution supply unit 6 and the ammonia selective reduction NOx catalyst 7. This hydrolysis reaction is represented by “(NH2)2CO+H2O→2NH3+CO2”.
Using the produced ammonia as a reducing agent, a selective catalytic NOx reduction is performed on the selective reduction NOx catalyst to purify NOx. Such reactions progress even if oxygen coexists, and 1 mole of ammonia (NH3) reacts per mole of nitric oxide (NO). Among such reactions, “NO+NO2+2NH3→2N2+3H2O” has the fastest reaction rate, and “4NO+4NH3+O2→4N2+6H2O” has the second fastest. There are other reactions such as “6NO+4NH3→5N2+6H2O”, “2NO2+4NH3+O2→3N2+6H2O”, and “6NO2+8NH3→7N2+12H2O”, but these reactions are relatively slow.
Due to the difference in the reaction rate of these chemical reactions, in the NOx purification system using the selective reduction NOx catalyst, the NOx purification performance is greatly influenced by the ratio between nitric oxide (NO) and nitrogen dioxide (NO2) that are coexisting gas in the exhaust gas supplied to the selective reduction NOx catalyst. FIG. 3 shows the NOx purification performances obtained as the result of simulated gas tests performed with different ratios between NO and NO2 coexisting in the supplied exhaust gas in a urea-SCR catalyst system. It can be seen that in the case of NO:NO2=50:50 (1:1), in which NO2 coexists (dotted line A), the NOx purification performance is improved over the whole catalyst temperature range, as compared to the case of NO:NO2=100:0 (1:0), in which NO2 does not coexist (solid line B). Especially, the improvement in the NOx purification performance is significant in the lower temperature range.
It is considered best that the NO:NO2 ratio is 50:50. However, the ratio of NO2 is extremely small in the NO:NO2 ratio in the exhaust gas emitted from diesel engines. This is one of the factors for degradation in the NOx purification performance in the low temperature range. Accordingly, inmost of the NOx purification systems using a selective reduction NOx catalyst, the NO2 ratio is increased to improve NOx purification performance in the low temperature range. The NO2 ratio is increased by oxidizing NO in exhaust gas, using an oxidation catalyst provided upstream of an ammonia solution supply unit such as a urea delivery valve.
Moreover, hydrocarbon (HC), being part of fuel, are supplied to the upstream (prior) oxidation catalyst from the engine through control of the in-cylinder fuel injection, and are oxidized by the oxidation catalyst. This allows the oxidation reaction heat to increase the temperature of the exhaust gas. Moreover, in conjunction with control of the temperature rise of the exhaust gas at lower temperature, the NOx purification performance in a lower temperature range is further improved.
However, if HC coexist in the exhaust gas in the oxidation reaction from NO to NO2 in the upstream oxidation catalyst, the reduction reaction of NO2 proceeds preferentially over that of the HC, reducing the generated NO2 back to NO. Accordingly, the increase of NO2 cannot be expected from the conventional NOx purification systems. This leads to a problem that the NOx purification performance does not improve. Additionally, in some cases, in a lower temperature range, HC are supplied from an engine to an oxidation catalyst to allow the oxidation reaction heat to increase the temperature of the exhaust gas. In such cases, HC that are not oxidized serve as a reducing agent, leading to a decrease of NO2. Thus, there is also a problem that the NOx purification performance in a lower temperature range is prevented from improving.
There has been proposed an internal combustion engine's exhaust purification device considering the NO:NO2=50:50, as described in, for example, Japanese patent application Kokai publication No. 2005-2968. The device includes a strong oxidation catalyst, an aqueous urea injection nozzle, and a SCR catalyst, starting from the upstream side of an exhaust system of the internal combustion engine. The purification device is provided with a switching valve for switching between an oxidation catalytic bypass for bypassing the strong oxidation catalyst and an exhaust gas passage. The switching valve causes the exhaust gas to flow into the oxidation catalytic bypass when the exhaust gas has a temperature at which the NO2 conversion rate of the strong oxidation catalyst is not less than 50%. Thereby, the device prevents generation of excessive NO2, which would cause the decrease in the NOx purification efficiency in the SCR catalyst. Furthermore, it has been proposed to provide the oxidation catalytic bypass passage with a weak oxidation catalyst that has a NO conversion rate of no more than 50%.
However, such exhaust purification device of an internal combustion engine needs to be provided with a parallel passage including the oxidation catalytic bypass and/or the weak oxidation catalyst, and with the valve for switching the passages. This may lead to a problem of an increase in size of the NOx purification system. Moreover, at the temperature less than a given temperature at which the exhaust gas does not pass through the oxidation catalytic bypass, the oxidation reaction heat in the oxidation catalyst cannot increase the exhaust gas temperature.